During the last few decades, there has been an overwhelming technological progress in electronics. Such advancement enabled mass production of electronic circuits and devices, which are orders of magnitude smaller and faster then their recent ancestors. However, today, conventional methods for production of elements approach rapidly the theoretical limits for miniaturization and speed. The primary goal of nano-electronics is to overcome the limitations of the common lithographic technique for fabrication of electronic circuits. There are two principal approaches for construction of nano-elements. The first, the so called, “top down” approach is actually an extension of the conventional approach for fabricating small elements by manipulation of bulk material. It is usually associated with refinement of existing techniques (even lithography) in order to produce yet smaller particles. The “bottom up” approach tries to construct nano-element form their atomic or molecular building blocks. It was initiated with the invention of the Scanning Tunneling Microscope (STM). This apparatus provides a way to observe and handle a single molecule and even a single atom, so that a nanoscopic element can be assembled atom by atom. The invention of the STM, although being a major breakthrough, still could not solve the problem of mass production on molecular scale. In fact, even the construction of a single complex nano-element, which may contain few thousands atoms, is sometimes too slow to be practical. There is, however, an attempt to solve this problem from a different perspective. This approach calls for the construction of simple elements, which will self assemble to create the complex structure (e.g., an electronic microcircuit). Important candidates for the role of the self assembled elements are DNA molecules.
A DNA molecule is made of two long polymer strands, which are attached to each other by relatively weak (and breakable) hydrogen bonds (H-bonds) between bases along strands. Each strand is made of units (or grains), composed of a sugar and one of the four possible bases. Phosphorus bridges (P-bridges) between the sugars connect the grains. The specific base sequence along the DNA strand determines its identity. The bases may only be connected in specific pairs (the ‘A’ base to a ‘T’ and a ‘G’ to a ‘C’) thus the identity of one strand determines the identity of the other. Modern biochemistry provides straightforward relatively inexpensive procedures for synthesizing DNA with any desired base sequence, and for amplifying this molecule to any desired quantity. Special enzymes can cut DNA strands at desired location identified by specific base sequence, or paste two strand segments together. Other enzymes catalyze polymer chain reaction (PCR) in which arbitrary large population of DNA molecules with the same base sequence are created from as few as only one sample molecule. The base sequence can be tuned now, so that new artificial pattern would be assembled spontaneously. To demonstrate this point, suppose that a, b, c and d are specific base sequences, and that the following strands types are synthesized: a b, b c, c d and dā (where ā is the complementary sequence of a etc.). It can be easily seen that these strands will assemble them self into a four-way junction pattern. More complicated patterns are produced in a similar manner, for example, only one exposed strand can be left at the ends of a double stranded molecule, to create a “sticky end” which may associate to complementary ends of other DNA molecules. The exact sequence of bases at each end serves as a specific code, which allows association only to the corresponding complementary code. By carefully designing these ends, complex structures, which self assemble from its DNA building blocks, can be produced. In this way, many molecular topologies, such as cubes, octahedrons and various knots, were realized. (see, for example, N. C. Seeman, Trends in Biotechnology, Vol. 17, (1999), p. 437, and references therein). Similar strategies are used in order to produce molecular micro-patterns. For example, two-dimensional lattices based on DNA, which have stripes patterns in the 10-nanometer scale were actually manufactured in the art.
Additional techniques were developed in order to manipulate a single DNA molecule. These methods involve the use of optical tweezers, Atomic Force Microscope (AFM), and various mechanical instruments such as glass micro needles, magnetic beads, etc. Using such techniques, DNA polymer may be stretched, twisted and separated into two single strands. The development of a method for coating a DNA strand with metal was a major step towards the goal of building DNA based electronic devices and circuits. For this purpose, the DNA is first attached at both ends to electrodes, which connect it to an external apparatus. After being coated, the molecule serves as a very thin conducting wire. This method may be modified to enable selective coating. Some known molecules, such as enzymes, recognize specific base sequences and attache to the DNA at these location. These molecules prevent coating in the region they occupy. After they are removed, some parts of the DNA molecule are left exposed.
An additional and much simpler technique for making conductive DNA molecules is to use molecules with only GC base pair (i.e. Poly-G Poly-C DNA molecules). Experiments, such as the one by Porath et al. Nature, Vol. 403, (200), p. 635, demonstrated that such molecules conduct current.
Doping the DNA molecule with acceptors and donors molecules can enhance the conductivity. Attachment of donors and acceptor to DNA is currently available process and was used as an experimental tool where the fluorescent response of the acceptor served as indication for electron transfer through the DNA.
Another additional technique for making conductive DNA molecules is to use M-DNA, which is a complex of DNA with divalent metal ions, such as, Zn2+, Co2+ or Ni2+. Upon addition of these metal ions, at pH conditions above 8, the pH decreases such that one proton is released per base pair per metal ion. It was demonstrated that M-DNA behaves as a molecular conducting wire by Aich et al., journal of molecular biology, 294 (2), 1999.
“DNA—Nanoelectronics: Realization of a Single Electron Tunneling Transistor and a Quantum bit Element”, The Sixth Foresight Conference on Molecular Nanotechnology, November 1998, discloses an idea of logical devices that are based on metal coated DNA SET transistor, however this article does not describe how to build such logical devices.
WO99/04440 “Microelectronic Components and Electronic Networks Comprising DNA” discloses a microelectronic network that is fabricated on a fibrous skeleton by binding or complexing electronically functional substances to the nucleic acid skeleton. The skeleton comprises fibers with nucleotide chains. The assembly of the fibers into a network is based on interactions of nucleotide chain portions of different fibers. However, it does not deal with the electrical properties of the DNA molecule itself.
Pat. No. WO99/60165 “Chemically Assembled Nano-Scale Device” discloses providing nano-scale devices, including electronic circuits, using DNA molecules as a support structure. DNA binding proteins are used to mask regions of the DNA as a material, such as a metal is coated onto the DNA. Included in the invention are DNA based transistors, capacitors, inductors and diodes. The present invention also provides methods of making integrated circuits using DNA molecules as a support structure. Methods are also included for making DNA based transistors, capacitors, inductors and diodes. However, as the first patent application, it does not deal either with the electrical properties of the DNA molecule itself.
All the prior art methods described above have not yet provided satisfactory solutions to the problem of providing commercially useful DNA-based single electron logical elements.
It is an object of the present invention to provide a method and apparatus for DNA-based single electron logical elements.
Other objects and advantages of the invention will become apparent as the description proceeds.